The present invention relates to spatial expansion of the apparent spacing between two stereophonic loudspeakers, and more particularly, to providing stereo sound without spatial expansion when the spaced-apart distance between a pair of loudspeakers is sufficiently large so that expansion is not desired.
Spatial stereo expansion in audio systems is well known in the art and has been available for many years. In such systems, the left and right channel signals are processed in a manner so that it appears to the listener that the distance of separation of the loudspeakers is much greater than the actual physical separation of the loudspeakers. This is called psycho-acoustic expansion. Examples of spatial stereo expansion are shown in U.S. Pat. No. 4,495,637 of Bruney, and U.S. Pat. No. 4,831,652 of Anderson. In such spatial expansion systems, a portion of the signal from the other channel is added in reverse phase, to the signal of the subject channel such that an ambience of spaciousness is introduced between the left and right channels. This ambience of spaciousness is independent of any separation in the original signal of the two channels. This feature has the desirable characteristic of making the acoustic perceived stereo image seem wider than the actual locations of a pair of stereophonic loudspeakers.
This is particularly desirable for the normal spacing of loudspeakers built into a cabinet of a television receiver or small radio, where the stereo speaker spacing between loudspeakers is typically only about 26-80 cm. apart. In such an arrangement, the perceived stereo effect is greatly improved with the perceived spatial expansion of the loudspeakers.
One well known method of increasing the perceived spatial expansion between stereophonic loudspeakers is by increasing the (L-R) signal level with respect to the (L+R) signal level when the signals are applied to the decoder matrix to form the L and R signals.
An undesirable effect of the spatial "expanded stereo" is that the (L-R) ambiance sound can be too loud compared to the (L+R) sound, often resulting in a center-stage vocalist between "drowned out" by the ambiance sound, or by the tonal variations between stereo and monophonic modes being large in extent. These objections, particularly the latter, can be more noticeable when using the auxiliary hi-fi output jacks sometimes provided in a TV sound system where the L and R signals can be applied to a high quality stereophonic reproduction system.
For the above reasons, it is desirable to provide a stereo signal without spatial expansion in a system which provides for the spatial stereo expansion when auxiliary loudspeakers are used.
Additionally, when auxiliary loudspeakers are used, i.e., loudspeakers disposed external of the television cabinet, these auxiliary loudspeakers can be placed at spaced-apart distances much greater in extent than the loudspeakers of the television receiver cabinet. For such a case, the rationale for using spatial expanded stereo is no longer applicable.